If you go with 4.10 you might see an increase in fuel mileage and a definite increase in power. With a 5 speed, 285/60/18 and 4.10 your RPM at 65 will be about 2200 to 2300, same setup auto trans RPM will be around 1900 to 2000. With the 4.56 you will see a power increase and the MPG could stay about the same or decrease. Same above setup 4.56 5 speed RPM will be around 2500 to 2600 and the auto would be around 2200 to 2300.